The present invention relates to a position detecting device, a liquid ejecting apparatus having the position detecting device, and a method of detecting the smear of a scale of the position detecting device.
There is known an ink jet printer as a liquid ejecting apparatus for ejecting a liquid to a predetermined medium of paper or the like. The ink jet printer is mounted with various motors of a sheet feeding motor for driving a carry roller for carrying printing sheet constituting a medium, a carriage motor for driving a carriage mounted with a printing head and the like. As such a motor, a DC motor is widely utilized with an object of calm sound formation or the like. The ink jet printer mounted with the DC motor includes a photosensor having a light emitting element and a light receiving element, and an encoder constituted by a scale alternately formed with a light transmitting portion for transmitting light from the light emitting element and a light interception portion for intercepting the light from the light emitting element as a position detecting device for carrying out a position control, a speed control or the like of the DC motor.
Further, according to the ink jet printer, it is known that when ink drops are delivered from a printing head, during a time period until the ink drops reach a printing face of a printing sheet or the like, or when the ink drops reach the printing face, ink mist floating in air by constituting portions of the ink drops in a mist-like form is produced and the produced ink mist is adhered to respective constitutions at inside of the printer. When the ink mist is adhered to a scale constituting the encoder, by an influence of the ink mist, the scale cannot pertinently transmit or block light emitted from the light emitting element. Hence, there is proposed an ink jet printer including a position detecting accuracy maintaining apparatus for maintaining a detecting accuracy of a linear encoder (refer to, for example, Patent Reference 1). Patent Reference 1 discloses a block plate arranged to block an interval between an ink ejection face of a printing head and a linear scale. Further, Patent Reference 2 discloses a constitution for correcting a duty ratio of an output signal outputted from a light receiving element becomes 50% even when the duty ratio is reduced by adhering ink mist.    [Patent Reference 1] JP-A-2005-81691 (refer to summary and paragraph No. 0032, FIG. 3 and the like)    [Patent Reference 2] JP-A-2004-202963 (refer to summary and paragraph Nos. 0034 through 0040, FIG. 3 through FIG. 5 and the like)
According to the linear encoder disclosed in Patent Reference 1, the detecting accuracy can be maintained by restraining the ink mist from adhering to the linear scale by the block plate. However, Patent Reference 1 does not propose specific means for detecting adherence per se of the ink mist to the linear scale (that is, smear of linear scale).
Further, although the constitution for correcting the signal is disclosed in Patent Reference 2, similar to Patent Reference 1, a degree of smear cannot be detected. Further, also in Patent Reference 2, with regard to the adherence of the ink mist to the rotary encoder, the adherence is not taken into consideration at all.
The invention has been carried out based on the above-described situation and it is an object thereof to provide a position detecting device including a scale capable of detecting a degree of smear and capable of preventing erroneous detection at a light receiving portion, a liquid ejecting apparatus including the position detecting device, and a method of detecting smear of a scale of the position detecting device.